The present invention relates, in general, to heterojunction field effect transistors, and more particularly, to heterojunction field effect transistors having controlled energy levels in a quantum well channel.
Field effect transistors operate by controlling current flow through a channel region with a gate electrode. To maintain current control it is necessary to confine charge carriers within the channel region. In metal oxide semiconductor FET (MOSFET) technology current confinement is accomplished by separating the channel region from the gate electrode by an insulating oxide region. In heterojunction FET (HFET) technology, however, the insulating region is not used and carrier confinement is achieved by a heterojunction barrier layer between the gate electrode and the channel region. In other words, the channel is formed by a quantum well using a material with a narrower bandgap than the barrier layer. A similar heterobarrier may be used below the channel region to keep charge carriers from straying into the substrate or buffer layer on which the channel region is formed. In HFET devices, the ability to confine charge carriers within the channel region is of great importance and directly affects device parameters such as pinch-off voltage and gate leakage.
To conduct current through the channel region charge carriers, holes for a P-channel device and electrons for an N-channel device must be provided in the channel region. Higher charge carrier concentration in the channel region results in higher transconductance and lower channel resistance in the HFET device. HFETs are usually modulation doped by placing a thin, heavily doped layer called a carrier supply layer in the barrier layer so that excess charge carriers tunnel from the carrier supply layer through the barrier layer to the quantum well channel region. Here, charge carriers are trapped in the quantum well. Increasing bandgap discontinuity between the carrier supply layer and the smaller bandgap channel layer deepens the quantum well and results in shallower quantized energy levels in the well. The shallow quantized energy levels can hold higher sheet carrier concentration and maintain better carrier confinement in the channel and improve the device characteristics.
In the past HFET devices have been made using gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) materials. GaAs and AlGaAs are closely lattice matched which allows composite devices to be formed. HFETs have been made using a GaAs buffer layer formed on a semi-insulating substrate, with a GaAs channel formed on the buffer layer. Al.sub.0.3 Ga.sub.0.7 As has been used for a barrier layer separating the GaAs channel from the gate electrode. This combination of materials results in a bandgap discontinuity of about 0.2 eV. While functioning devices have been formed with these materials, the relatively low bandgap discontinuity did not confine carriers to the channel region very well.
Recently, a great deal of interest has been expressed in the use of strained layer, narrow bandgap indium gallium arsenide (InGaAs) to replace gallium arsenide as a channel material in HFETs. The maximum allowable thickness of the InGaAs strained layer as well as the indium arsenide (InAs) mole fraction depends on the lattice misfit between a GaAs substrate and InGaAs channel layer. Best device results have been obtained within InAs mole fraction of 25% giving a bandgap discontinuity of approximately 0.33 eV for Al.sub.0.3 Ga.sub.0.7 As/In.sub.0.25 Ga.sub.0.7 As compared to the Al.sub.O.3 Ga.sub.0.7 As/GaAs bandgap discontinuity of only 0.2 eV. Because thickness and indium arsenide mole fraction of an InGaAs channel region are limited due to crystallographic concerns, further improvement using these material systems has not been expected.
Accordingly, it is an object of the present invention to provide a heterojunction field effect transistor having improved pinch-off voltage.
Another object of the present invention is to provide an HFET device having reduced gate leakage.
Still another object of the present invention is to provide an HFET structure having mono-atomic barrier and well layers formed in a channel region to modify energy levels within a channel region quantum well.
Still another object of the present invention is to provide an HFET device with improved carrier concentration in the channel region.
A further object of the present invention is to provide an HFET device having improved charge carrier confinement within the channel region.